Vacuum motors are used for actuating valves or vents in heating, ventilating and air conditioning (HVAC) control systems in motor vehicles, and various other applications. Vacuum motors, such as the one described in U.S. Pat. No. 3,613,513 to Johnson, transfer vacuum pressure into linear motion. Typically, a vacuum motor includes a housing and a reciprocal plunger, which is connected generally by a linkage mechanism to a vent or other HVAC device for actuation. A collapsible bladder or bellows is connected to the plunger within the motor housing. The bellows forms an airtight compartment in the housing. A helical or cylindrical spiral spring is used to bias the bellows to a fully expanded position. The motor is connected to the air line of the HVAC control system. A negative pressure or vacuum in the air line causes the bellows to collapse and compress the spring. As the bellows collapses, the attached plunger is pulled axially into the motor housing. The movement of the plunger shifts the connected vent or otherwise actuates the connected HVAC device. When the pressure within the bellows is equalized through the air line, the spring tension expands the bellows to extend the plunger. The extension of the plunger returns the connected vent to its original position or further actuates the HVAC device.
Improvements in vacuum motors have centered around attempts to reduce the size of the motor housings without decreasing the operational stroke of the plungers. Conventional vacuum motors are usually constructed of metal, which is costly, relatively heavy, and difficult to fabricate and assemble. Reducing the size of the housings can reduce the production costs of the motors. The dimensions of conventional housings are limited in part by the dimension of the cylindrical spiral springs used to bias the bellows. In a cylindrical spiral spring, each turn or coil of the spring overlies another. When the spring is fully compressed, its coils abut against each other. Consequently, the minimum collapsed height of a spring is limited to the band width of each coil multiplied by the number of coils in the spring. This minimum collapsed height of the spring adds additional size to the motor housing without any increase to the operational stroke of the plunger. Typically, the solution to this dimension problem was to use cylindrical springs with high spring coefficient (K) values and fewer coils. Decreasing the number of coils decreases the life of the motor and increases various other operational problems. For example, springs with fewer coils tend to bow outwardly during compression. Such bowing of the springs force the bellows' side walls into contact with the side walls of the housing. This contact increases the operational noise of the motors and may damage the bellows.